SPECIAL ISSUE
Bowing of dimensional granitic stones
P. Va´zquez • S. Siegesmund • F. J. Alonso
Received: 12 August 2010 / Accepted: 16 November 2010 / Published online: 3 December 2010
 The Author(s) 2010. This article is published with open access at Springerlink.com
Abstract Bowing is a well-known phenomenon seen in
marbles used as building veneers. This form of rock
weathering occurs as a result of external factors such as
temperature, humidity, the system for anchoring the marble
slabs or the panel dimensions. Under the same external
conditions, many factors will determine the degree of
deformation including petrography, thermal properties and
residual locked stresses. The usual way to solve the prob-
lem of bowed marble slabs is to replace them with other
materials, such as granites, in which the deformation still
exists but is less common. In this study, eight ornamental
granites with different mineralogy, grain size, grain shape,
porosity and fabric were tested in a laboratory to assess
their susceptibility to bowing. Three slabs of granite, each
cut with a different orientation, were studied under dif-
ferent conditions of temperature (90 and 120C) and water
saturation (dry and wet) to investigate the influence of
these factors together with that of anisotropy. At 90C,
only the granite with the coarsest grain size and low
porosity exhibited deformation under wet conditions. At
120C and wet conditions, three of the granites showed
evident signs of bowing. Again, the granite with the
coarsest grain size was the most deformed. It was con-
cluded that the wide grain size distribution influences mi-
crocracking more than other expected factors, such as the
quartz content of the rock. Also, mineral shape-preferred
orientation and porosity play an important role in the
bowing of the studied granites.
Keywords Granitoids  Bowing  Texture 
Thermal expansion
Introduction
Bowing is one of the most spectacular deterioration fea-
tures seen in dimensional stones used in fac¸ades. The
phenomenon is well known in marbles, having been
observed in various scenarios from ancient gravestones
(Grimm 1999) to modern buildings such as the Amoco
building in Chicago, Oeconomicum building in Go¨ttingen
(Koch and Siegesmund 2002) or Finlandia Hall in Helsinki
(Ritter 1992). A more recent summary of marble bowing is
given by Siegesmund et al. (2008a), in which the latest and
the most important hypotheses debated in scientific litera-
ture are presented. Siegesmund’s comparative case study is
based on three different public buildings, which are clad
with Peccia marble, Rosa Estremoz and Carrara marble,
respectively.
Marbles may show bowing after a short period of
exposure due to the anisotropic behaviour of calcite crys-
tals with thermal variations. When heated, calcite crystal
expands (26 9 10-6/C) parallel to the crystallographic
c-axis and contracts (-6 9 10-6/C) in a direction normal
to it (Kleber 1990). Historically, some researchers have
attributed the bowing exclusively to the effects of repeated
heating–cooling cycles (Sir Rayleigh 1934; Rosenholtz and
Schmidt 1949; Ondrasina et al. 2002). Logan et al. (1993)
and Logan (2004) suggested that the bowing of the marble
slabs in the Amoco building was due to the anomalous
expansion–contraction behaviour of calcite combined with
P. Va´zquez (&)  F. J. Alonso
Department of Geology, University of Oviedo,
Jesus Arias de Velasco s/n, 33005 Oviedo, Spain
e-mail: pvazquez@geol.uniovi.es
S. Siegesmund
Geoscience Centre, University Go¨ttingen,
Goldschmidtstrasse 3, 37077 Go¨ttingen, Germany
123
Environ Earth Sci (2011) 63:1603–1612
DOI 10.1007/s12665-010-0882-y
the release of locked residual stresses. Scheffzu¨k et al.
(2007) performed bowing tests on Carrara marble together
with measurements of lattice-preferred orientation (LPO),
residual strain and thermal expansion, and concluded that
the so-called locked-in stress had an important impact on
the bowing of marbles.
Winkler (1994, 1996) explained the expansion and
warping phenomena as the result of stress relief aided by
hygric action, whilst Bortz et al. (1988) stated that the
deformation was due to variations in moisture content.
Koch and Siegesmund (2002, 2004) and Grelk et al. (2004)
agree that a combination of these factors is necessary for
bowing to occur ‘‘…repetitive heating–cooling under dry
conditions leads to considerable inelastic residual strain
only in the first thermal cycle. The residual strain contin-
uously increases again if water is present, whereby the
moisture content after a thermal cycle has a certain impact
on the decay rate’’ (Koch and Siegesmund 2004).
Microstructure-based, finite element simulations were
used by Weiss et al. (2002, 2003); Saylor et al. (2007) and
Shushakova et al. (2010) and provided an excellent insight
into the magnitude and mechanisms of thermal degrada-
tion. It is evident that grain size, shape, fabric, interlocking
and crystallographic-preferred orientation are all important
(see also the discussion in Zeisig et al. (2002)). In situ
observation reported by Luque et al. (2010) supported the
results of the modelling done by the earlier researchers
above.
Bowing is not related only to marbles. Grimm (1999)
and Siegesmund (2008) reported that slabs made of granite,
conglomerate or limestone may also show bowing. Winkler
(1996) referred to warping in granite cemetery slabs, which
had been installed only 20 years previously. Continuous
exposure to almost 100% relative humidity (RH) and
higher temperatures inside the tomb cavity had expanded
the stone slabs more on the inside than on the outside,
causing bowing. Siegesmund et al. (2008b) studied the
bowing observed at the Maxi market in Ljubljana (Slove-
nia) built with Cezlak grandiorite (Fig. 1) and concluded
that different volumetric expansions of the rock-forming
minerals may have led to the creation of both locked-in and
thermal stresses in the panels, which in turn may have
caused the bowing. Generally, in cases of marble bowing,
the industrial solution is to replace the deteriorated panels
with granite slabs; however, the research above suggests
that prior to doing so the granites themselves should also
be investigated with respect to their thermal expansion
behaviour.
In summary, there are numerous factors that influence
marble bowing despite its simple mineralogical composi-
tion. Moreover, even if only a single factor is considered,
the conclusions may be contradictory depending on
which materials are compared. Therefore, to evaluate the
deformation in heterogeneous materials such as granites,
the number of variables to be taken into account will be
higher.
The main aim of this study was to assess the bowing
potential of various granite types with respect to their
mineralogical composition and rock fabric. For that pur-
pose, ageing tests were carried out in a laboratory under
different conditions of moisture and temperature. Investi-
gations from buildings suggest that laboratory tests can be
used to simulate the decay observed in natural environ-
ments (see summary in Siegesmund 2008). From the
results obtained, the influence of temperature and moisture
variation on granite deformation was analysed. The
objectives were to know which of the main petrographic
characteristics play a key role in bowing and whether or not
the mechanism of marble deformation also applies to
granites.
Materials
Eight granitoids used internationally as dimensional stones
were investigated. All of them differed in their mineral-
ogical composition, grain size, rock fabric, porosity and
weathering degree. Their commercial names are Albero
(A), Grissal (G), Gris Alba (GA), Negro Galicia (NG),
Rosavel (R) and Rosa Porrin˜o (RP), all from Spain; Golden
Ski (GS) from Portugal; and the ortogneiss Multicolor Red
(RM) from India (Fig. 2).
The petrographic characteristics of the eight granitoids
are given in Table 1. Mineral proportion and grain size
were studied using optical polarisation microscopy (MOP)
and digital image processing (DIP). Grain boundary shapes
and orientation, and the systems of microcracking, were
also assessed using MOP. Basic physical properties, which
may influence the bowing, were measured. Density and
Fig. 1 Cezlak granodiotite showing bowing
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123
Fig. 2 Macrophotos of the selected granites. a Rojo Multicolor, b Negro Galicia, c Gris Alba, d Grissal, e Rosa Porrin˜o, f Rosavel, g Golden
Ski, h Albero
Table 1 Main characteristics of the selected granitoids
Trade name Petrographical classification Modal content in vol. (%) Grain size (mm)
Q KF P M Q KF P M Avrg.
Rojo Multicolor (RM) Ortogneiss 10 40 50 \1 \1 \1 \1 \1
Negro Galicia (NG)a Tonalite 18 37 24 5 4 2.5 3
Gris Alba (GA) Monzogranite 23 37 23 17 5 5 4 2 4
Grissal (G) Monzogranite 25 35 32 8 8 18 9 2 11
Rosa Porrin˜o (RP) Syenogranite 29 49 13 9 10 17 7 3 12
Rosavel (R) Quartzsienite 12 60 20 8 10 33 14 3 25
Golden Ski (GS) Monzogranite 43 27 18 12 4 4 4 2 4
Albero (A) Granodiorite 35 10 30 25 5 5 6 4 5
Trade name, Streckeisen classification (1976), mineral percentage and grain size
Q quartz, KF alkali feldspar, P plagioclase, M mica
a NG has amphiboles and pyroxenes completing the percentage with 2.5 cm of average size
Environ Earth Sci (2011) 63:1603–1612 1605
123
open porosity were obtained following the UNE-EN 1936
standard. Linear thermal expansion coefficients (a) were
determined in three normal directions (X and Y, parallel to
the foliation plane, and Z normal to it) using a dilatometer
(Strohmeyer 2003) (Table 2).
With regard to the granitoids’ mineralogy (see Table 1),
RM (Fig. 2a) is an orthogenesis with alkali feldspar quartz-
syenitic composition. Rosavel (Fig. 2f) is a porphyritic
quartz syenite with lower quartz content and the highest
proportion of perthitic potassium feldspar phenocrysts
amongst all the studied stones. RP (Fig. 2e) is a syenog-
ranite also with a higher proportion of alkali feldspar than
plagioclase and with a high quartz content. GA, G and GS
(Fig. 2c, d and g, respectively) are monzogranites with
similar proportions of alkali and plagioclase feldspars.
Amongst them, GS has the highest quartz and the lowest
feldspar content. GA is a two-mica monzogranite with a
muscovite:biotite proportion approximately of 2:1. Albero
(Fig. 2h) is a two-mica granodiorite with a high content of
mica, quartz and plagioclase. NG (Fig. 2b) is a tonalite
with about 20% of amphiboles and pyroxenes.
The orientation of the XYZ axes are as follows: Z is the
direction normal to the foliation plane defined by the main
crack system and mica plane orientation in the quarry,
which corresponds to the saw plane. X and Y were not
oriented with respect to the quarry, so when the granite had
a directional characteristic by which it could be oriented
(for example, a mineral or crack alignment), the orientation
was designated to be the direction of the X axis. In cases
where no visible orientations could be discerned, the
direction of X was established at random. GA, GS and
Albero exhibit mineral shape orientation in the XY plane,
parallel or slightly diagonal to the X axis (see Fig. 2c, g, h,
respectively). NG presents the mica and Rosavel the K-
feldspars oriented parallel to the Y axis. RP shows the main
crack system parallel to the X axis and the secondary one
parallel to the Z axis. In Grissal, without any macroscopic
rock fabrics, the X and Y axes were selected randomly. In
RM, the XY plane shows folded foliation, whilst the other
two planes exhibit mineral orientation parallel to the axial
planes of the folds (Fig. 4b).
RM (Fig. 2a) has a metamorphic fabric. The plane that
corresponds to the industrial sawing plane shows folded
foliation. This rock has the smallest grain size amongst the
granites studied, although isolated alkali feldspar pheno-
crysts, commonly of millimetric size, can be found. RM
shows irregular grain interlocking due to metamorphic
processes. NG, GA and GS (Fig. 2b, c, g, respectively) are
fine grained granites with evident mica orientation. They
differ in mineralogy and porosity. GA and NG have hyp-
idiomorphic to allotriomorphic minerals with irregular
boundaries in quartz, whilst most of the minerals in GS
present idiomorphic shapes. Albero (Fig. 2h) is also fine
grained, but coarser than NG, GA and GS, and with
elongated allotriomorphic minerals following the foliation.
The coarse-grained Grissal (Fig. 2d) and RP (Fig. 2e)
rocks differ in their alkali feldspar type. No macroscopic
foliation can be observed in Grissal. The fundamental crack
system of RP can be observed in the quartz grains with two
crack orientations, one that corresponds to the foliation
plane and another one normal to it. Rosavel (Fig. 2f) is a
porphyritic granite with alkali feldspar phenocrysts up to
60 mm in length. Feldspars in Grissal, RP and Rosavel are
idiomorphic with straight grain boundaries, whilst the
quartz in these rocks show straight and interlobate grain
boundaries. The shorter a-axis of K-feldspars in Rosavel is
observed almost normal to the foliation plane (Va´zquez
2010).
Most of the stones have low open porosity, with values
between 0.5 and 1%. All of them are ‘quarry fresh’, and the
most common weathering patterns are microcracks in
quartz and sericitization in plagioclase. Albero and GS
show a notable initial weathering with the presence of clays
and open transgranular cracks, and consequently the
highest porosity amongst the studied granites.
The most porous granites (Albero and GS), together
with Grissal, show the highest average thermal expansion
(Table 2), whilst GA, RP and RM show the lowest.
Regarding anisotropy, GA, Grissal, RP and Rosavel exhibit
the lowest thermal expansion perpendicular to the foliation
plane (Z), and RM exhibits the highest.
Bowing test
The bowing tests were carried out according to the Nord-
test Method NT BUILD 499 (2002). Three slabs of each
granite were placed in a bed of grit at a distance of 50 mm
to the heating system (Fig. 3). The slabs were sawed in
Table 2 Basic physical properties of the selected granitoids
Trade name U
(%)
q (kg/
m3)
a (10-06/8C)
X Y Z Average
Rojo Multicolor
(RM)
0.6 2,640 6.74 6.70 8.15 7.30
Negro Galicia (NG) 0.5 2,830 8.37 8.60 8.75 8.53
Gris Alba (GA) 1.2 2,630 7.89 7.80 7.36 7.76
Grissal (G) 0.7 2,640 9.54 10.10 8.63 9.83
Rosa Porrin˜o (RP) 1.1 2,600 7.94 7.25 6.52 7.59
Rosavel (R) 0.9 2,600 8.98 8.87 6.98 8.59
Golden Ski (GS) 2.4 2,560 9.89 9.02 9.12 9.70
Albero (A) 5.3 2,505 9.36 9.81 10.30 9.91
Trade name, open porosity (U), apparent density (q) and linear
thermal expansion (a)
1606 Environ Earth Sci (2011) 63:1603–1612
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different directions in relation to the rock fabrics to char-
acterise the anisotropic behaviour of the rock materials
(Fig. 4). In this study, the XY plane coincided in all the
granites (except for RM) with the foliation plane, which
related also to the preferred sawing direction in the quar-
ries. The slab dimensions were 40 9 10 9 1.5 cm. In
recent years, 3 cm has been the typical thickness for a
fac¸ade panel in industrial practice, however, the current
trends in building stone manufacturing are to produce
thinner slabs. In addition, Grelk et al. (2004) and Sieges-
mund et al. (2008b) observed a higher degree of bowing in
thinner slabs.
In this study, the test performed included 36 heating–
cooling cycles. To assess the influence of moisture and
temperature on the deformation of the different granites,
the test was divided into four stages of nine repetitive
cycles. Heating took 6 h followed by 18 h of cooling at
room temperature:
• Stage 1: heating up to 90C in dry conditions;
• Stage 2: heating up to 90C in wet conditions;
• Stage 3: heating up to 120C in dry conditions;
• Stage 4: heating up to 120C in wet conditions.
The wet conditions were obtained by adding water to the
grit bed until it reached about 5 mm from the bottom of the
slabs; this gave a wet bottom surface and a dry upper
surface (Siegesmund et al. 2008b).
The bowing was measured after each cycle with a
measurement bridge (accuracy ± 1 lm/35 cm). The zero
position was calibrated using a core of silica glass with
coplanar surfaces. The deformation was detected in mm/m,
calculated according to the circle equation of Koch and
Siegesmund (2004), where positive values indicate convex
bowing and negative values indicate concave bowing
(Fig. 5).
Results
The results of the bowing test are given in Fig. 6, with the
outcome of each third cycle shown.
During the first stage, with heating up to 90C and dry
conditions, the eight granites hardly exhibited any defor-
mation. Grissal and the porous granites, Albero and GS,
showed the greatest concave bowing, between 0.3 and
0.35 mm/m. Amongst all the samples, only GS presented
convex deformation (in the ZY slab) with maximum values
in the second cycle (0.5 mm/m). In the second stage, with
the same temperature but adding water to the bottom sur-
face, the trend in deformation varied. Four of the granites,
GA, NG, Albero and RM, became concave in the first cycle
of the stage, but thereafter, the general deformation of all
the stones was to convex shapes. Only Rosavel displayed
remarkable behaviour, by reaching values up to 0.7 mm/m,
Fig. 3 Experiment set up with some of the slabs
Fig. 4 Orientation of the slabs.
XY corresponds to the foliation
plane. a Grissal. b Rojo
Multicolor with a different
foliation plane than the
industrial one. The slabs are
named as the plane in which
they are included
Environ Earth Sci (2011) 63:1603–1612 1607
123
which was significantly higher than the values measured in
the rest of the stones. In the third stage, with the increase of
temperature and dry conditions, almost all the samples
showed concave or no further deformation. Only GS and
Albero in the ZY slab continued with convex bowing,
although the values of deformation were very low at 0.13
and 0.18 mm/m, respectively. In the fourth stage, all the
slabs bent in a convex direction and the deformation values
for most of them were comparable to those shown
throughout the earlier stages. However, three of the gran-
ites exhibited considerable bowing. Grissal and RP showed
similar bowing in all the slabs, up to 1 and 0.7 mm/m,
respectively. Rosavel was the granite that experienced the
highest, and clearly anisotropic, bowing; The ZY slab
Fig. 5 Convex and concave deformation in relation with bowing
values and placement in the building
Fig. 6 Bowing of the eight ornamental granites along the four
different stages, with nine cycles in each stage. Each third cycle is
drawn. The three different orientations are shown. The abrupt change
in some of the granites in the last stage with the most extreme
conditions evidence the bowing potential of these stones
1608 Environ Earth Sci (2011) 63:1603–1612
123
presented deformation of up to 3 mm/m at the end of the
test, whilst the other two directions had final values of
about 1 mm/m. At the end of the test, in Rosavel clay
minerals were observed filling cracks in the upper face.
In summary, after all four stages of the test, three groups
can be distinguished:
1. Granites that hardly showed any bowing: RM, NG and
GA. These three granites have mineral shape-preferred
orientation, small grain size and low porosity. They
suffered bowing, but it was not comparable to the rest
of the granites.
2. Granites that experienced bowing only in wet condi-
tions and more evidently with high temperature: G, RP
and R. These granites have the coarsest grain size of all
the granites, combined with low porosity. They
showed a pronounced change in the last stage of the
test, deforming to convex shapes. In this group, only
Rosavel exhibits mineral shape-preferred orientation
(feldspars).
3. Granites that showed high absolute deformation, but
no progressive tendency: Albero and GS. Both are
weathered granites with mineral shape-preferred ori-
entation, high porosity and the presence of clays. The
graphs below (Fig. 6) show that the granites both
followed a deformation trend that did not change with
temperature or between dry and wet conditions.
Discussion
The bowing of stones is considered to be produced as a
result of anomalous mineral expansion in response to
temperature and moisture variations. Thermal expansion is
a more complex process in granitoids than it is in marbles
due to the presence of different mineral phases and fabrics.
During heating and cooling, temperature differentials
between the different minerals, combined with differing
thermal mineral expansion values, may initiate cracks or
cause existing cracks to grow or propagate, as was sug-
gested by Siegesmund et al. (2008b) and also observed in
microscopic studies by Mauko et al. (2006).
Within the major rock-forming minerals of the investi-
gated granitoids (see Table 1), the expansion in volume of
quartz and mica is about four times higher than that of
feldspars with the following values [in 10-6/K]: 37.0
quartz, 35.4 muscovite, 9.7–15.6 K-feldspar, 8.9–15.4
plagioclase and 6.6 albite (data from Strohmeyer 2003 and
Fei 1995). This contrast may cause internal stresses to build
up during heating, leading to the development of micro-
cracks in quartz boundaries and the opening of cleavage
planes in mica, as Siegesmund et al. (2008b) have dis-
cussed. Volume expansion is further enhanced when
minerals with anisotropic thermal expansion show a crys-
tallographic orientation. This is highly relevant for quartz
and mica, which, in addition to demonstrating the highest
linear thermal expansion of all the minerals in the inves-
tigated granitoids, also show the most anisotropic values,
with coefficients in quartz of a = 7.7 9 10-6/K parallel to
the c-axis and 13.3 9 10-6/K perpendicular to it, whilst in
mica these values are 17.3 9 10-6/K and 9.65 9 10-6/K,
respectively (Siegesmund et al. 2008b). For the purpose of
comparison, Devore (1969) indicated that for increments in
temperature up to about 100C, the highest alkali feldspar
expansion (a-axis) is similar to the smallest quartz expan-
sion (parallel to the c-axis). It seems likely, therefore, that
granites with high quartz and mica content will show high
thermal expansion (Castro de Lima and Paraguassu´ 2004).
In agreement with this, Albero and GS, which have the
highest quartz and mica content of the eight test materials,
did display the highest thermal expansion. However, two of
the test stones did not conform with this theory; Rosavel
displayed a high thermal expansion coefficient, yet it
contains the lowest quartz content, and RP displayed one of
the lowest thermal expansion coefficients despite having a
high quartz content (see Table 1, 2).
The determination of the linear expansion coefficient in
three directions demonstrated anisotropic behaviours in
most of the granites (see Table 2). RM, GA and RP had the
lowest thermal expansion. GA, which showed mica ori-
entation, is considered to be thermally isotropic (anisotropy
index A = 0.07 with Aa = (amax-amin)/amax). RM and RP
were significantly anisotropic (both with A = 0.18). RM
shows mineral shape-preferred orientation parallel to the
Z axis (Fig. 4b), and consequently had the highest expan-
sion in this direction of all the granites studied. In contrast,
RP exhibited the lowest expansion in the Z direction, which
can be explained by the presence of oriented inter- and
intra-granular cracks perpendicular to Z, which may have
minimised the expansion. Amongst them, only RP, the
granite with the coarsest grain size and the highest quartz
content, showed clear bowing, although with slightly
higher values in the Z direction (Abowing = 0.13 where
A = (Abowing max-Abowing min)/Abowing max) (Fig. 6). NG
and Rosavel had thermal expansion values that were
intermediate amongst the granites studied. They have in
common, a low quartz content (18 and 12%, respectively)
and mineral shape-preferred orientation. NG is a fine grain-
sized tonalite with mica orientation parallel to the foliation
plane; however, its thermal expansion is isotropic
(Aa = 0.04) and no bowing was observed. Rosavel pre-
sents alkali feldspar phenocrysts parallel to the foliation
plane, and consequently had the lowest expansion and
bowing in the Z direction (Aa = 0.22 and Abowing = 0.61).
Figure 7 shows that Rosavel0s XY plane slab, parallel to the
foliation, barely presents any mineral shape-preferred
Environ Earth Sci (2011) 63:1603–1612 1609
123
orientation. The slab with the greatest bowing followed the
YZ plane, where K-feldspar long axis were parallel to the
short side of the panel (Y). Albero, GS and Grissal (G) had
high values of linear thermal expansion coefficient and all
of them showed evident deformation, although only Grissal
exhibited a clear bowing (Fig. 6). Albero and GS have the
highest content of all the granites studied in both quartz and
mica, and their thermal expansion values are slightly
anisotropic (both with Aa = 0.09 and Abowing = 0.50
higher in the X direction). Grissal, which does not exhibit
any visible orientation, has anisotropic thermal expansion
(Aa = 0.15) with the lowest values in the Z direction;
moreover, its bowing was a bit higher in that direction
(Abowing = 0.22) (Fig. 4a). It seems that most of the slabs
in which the thickness corresponds to the direction with the
lowest thermal expansion coefficients experience more
deformation than slabs in which this is not the case.
Therefore, to explain the bowing mineral, expansion alone
must not be taken into account. The three granitoids that
showed the highest bowing can be grouped according to
their different values of thermal expansion (low, interme-
diate and high).
Other parameters that influence marble bowing, and that
apply to granites also, are grain size, differences in grain
size distribution and grain boundaries. Go´mez-Heras et al.
(2006) affirm that ‘‘…crystal size is shown to be a major
control on surface temperature differences between min-
erals’’. He also attributed the generated stresses to the fact
that ‘‘…stones with large differences in mineral size might
be expected to experience magnified near surface stresses
due to thermal expansion differences’’. Siegesmund et al.0s
research (2008b) supported this theory when it obtained
higher bowing in coarser grain-sized granite with larger
phenocrysts. In this study, only the three granites with the
coarsest grain size and also with straight grain boundaries
(G, RP and R) exhibited bowing when temperature and
moisture acted together. Amongst them, Rosavel, with the
coarsest grain size and large differences between the size of
the alkali feldspars and the rest of the minerals, showed the
highest bowing of all the three slabs, reaching up to 3 mm/
m during the test (Fig. 6).
Bowing is influenced by external factors such as tem-
perature and moisture. When the selected granitic stones
were exposed to temperature increases up to 90 and 120C
in dry conditions, deformation was hardly evident (Fig. 6).
The most common deformation was concave bowing
(Fig. 5), probably due to the fact that the upper surface was
exposed to a higher and more extreme temperature varia-
tion than the bottom one, and consequently a higher quartz
contraction during cooling (Winkler 1996). In most of the
cases when water is added, the trend turned into convex
deformation. The mineral dilation in contact with water is
strong, so the bottom surface suffers more contraction than
the upper one. Malaga et al. (2008) indicated that ‘‘…crack
propagation in wet rocks is more suitable because it takes
more energy to replace a solid–solid contact with a solid–
air contact than to replace it with a solid–water contact’’.
Only three granites did not show this trend: Albero, GS and
NG. During the test, Albero and GS, which have the
highest porosity amongst the stones, were water saturated
in minutes due to capillary uptake, which means that the
differences between the two faces were not as extreme as in
the rest of the granites. NG showed a strong change to more
concave shapes in the first cycles when the water was
added and constant values during the rest of the wet cycles.
Other factors may also influence granite deformation;
for example, residual stresses, slab thickness and albedo.
Residual stresses, caused by thermal stresses amongst
minerals during the uplift and cooling of the granitic pluton
(Vollbrecht et al. 1991), are thought to play an important
role later in microcracking during heating and cooling
cycles (Siegesmund et al. 2008b). With thinner slabs, there
is likely to be a higher degree of bowing although this
variable was not specifically tested within the scope of this
study. Go´mez-Heras et al. (2006) and Hall et al. (2005)
called attention to the influence of mineral albedo. As Hall
(2010) pointed out, light transmissive minerals change the
Fig. 7 K-feldspar orientation in R granite. XZ and ZY planes show
the most quantity of oriented crystals with 92 and 65 measurements,
respectively. In the XY slab, only 18 oriented K-feldspar were found.
The slab with the greatest bowing followed the YZ plane and showed
K-feldspar parallel to the Y axis
1610 Environ Earth Sci (2011) 63:1603–1612
123
thermal response in the outer millimetres of the rock,
creating thermal stresses when in contact with opaque ones.
In laboratory conditions, the temperature reached by every
mineral depends only on the thermal properties and inter-
relation with the surrounding minerals. However, when
exposed outdoors, minerals with lower albedo heat up more
than the lighter ones due to insolation. This enhances the
stresses amongst minerals in non-laboratory conditions.
Suzuki et al. (1995) demonstrated that physical and
chemical weathering both contribute to crack growth. They
showed this by studying changes in granite properties after
long-term immersion in hot water at 90C. They observed
the formation of clays on the surface of the plagioclase and
around microcracks after long periods of water immersion.
At the end of the test in this study, Rosavel, which had the
greatest bowing, exhibited clays that filled microcracks on
the external slab face, i.e. the side which was not in contact
with water, despite the fact that no clays were observed in
the initial stone. The formation of clays, or the migration of
existing ones towards the upper surface, may also produce
higher stresses than those existing on the bottom surface
and thus may contribute to convex deformation. The
duration of the test in this study was lower than that of the
test carried out by Suzuki et al. (1995). However, the water
temperature in both tests was over 100C, which means
that both studies tested the effects of liquid water and also
water vapour. The presence of water in two different states
may produce a different chemical environment, which may
consequently create the right conditions for the occurrence
of reactions that may generate clays. Microcracks increase
when there are variations in temperature, which helps the
clays to rise to the surface of the rock and to produce
pressure on the crack walls.
Conclusions
This study of the bowing potential of ornamental granites
resulted in the following conclusions:
– Granites may show bowing with values comparable to
marbles when temperature and moisture act together.
– Usually, slabs that have the lowest thermal expansion
coefficients normal to the slab plane tend to suffer the
highest deformation.
– Porosity is one of the features that most affects
deformation. Slabs with high porosity showed defor-
mation, but not progressive bowing. This may be
attributed to low differences in temperature and
humidity between slab faces due to the fast water
capillary uptake.
– Mineralogy also influences bowing, but is not the most
important factor. For any given material, its mineral
proportion, size distribution of different mineral phases
and the above-mentioned porosity and thermal expan-
sion values must be taken into account altogether.
– In this study, the key factor that determined the degree
of bowing was the coarse grain size, whilst the
anisotropy of the deformation was defined by the
mineral shape-preferred orientation. However, both
factors must be considered with care. Granites are
heterogeneous materials and it is difficult to extrapolate
their behaviour accurately even with several similari-
ties between materials.
Acknowledgments We would like to thank the Ministry of Science
and Innovation for their support of project MAT2004-06804-C02-01,
and the FICYT for their support of project IB-09-080. Our thanks are
also extended to Dr. Axel Vollbrecht for his advice and to Manuela
Morales for her help during the laboratory work.
Open Access This article is distributed under the terms of the
Creative Commons Attribution Noncommercial License which per-
mits any noncommercial use, distribution, and reproduction in any
medium, provided the original author(s) and source are credited.
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